1. Field of the Invention
The subject invention relates to fittings for high pressure and vacuum fluid lines used in environments where repeated connections and disconnections are necessary.
2. Description of the Prior Art
Gaseous fluids at pressures of 3,500 psi. and higher are used in many manufacturing processees, including the manufacture of microprocessors and other electronic components. High pressure gases also are used in various chemical engineering processes, in medical applications and in machining or welding operations. The particular gaseous fluid may vary from one industrial application to the next.
Gas for industrial applications typically is delivered to the manufacturing site in gas containers which are appropriately constructed to contain the high pressure gas therein, and which can be selectively connected and disconnected into a manufacturing system.
Purity of the specified gas often is critical to the manufacturing process. Thus, it is important for the supplier of high pressure gas to supply the gas in containers that prevent transfer leaching of contaminants from the container and/or from the attached fittings into the gas stream. It is also necessary to prevent diffusion or leakage of atmospheric contaminants through the gas delivery system and into the gas stream.
The typical prior art fitting for high pressure fluid applications includes a first generally tubular component having a fluid passage extending therethrough and having an array of external threads. A second tubular component with a fluid passage extending therethrough is dimensioned to mate in end-to-end relationship with the first component. The second component, however, generally will not include threads, but rather is provided with a pressure bearing flange or shoulder extending outwardly in proximity to the mating end. A coupling nut with an array of internal threads is dimensioned to threadedly mate with the external threads on the first tubular component of the fitting. The coupling nut further includes an inwardly extending flange dimensioned for engaging the pressure bearing flange of the second tubular component. The prior art fitting is connected by aligning the first and second components in end-to-end axial relationship to one another. The nut is then threadedly engaged with the first component. Sufficient tightening of the nut draws the abutting ends of the aligned components into tight engagement with one another.
Nonmetallic fittings and nonmetallic washers or gaskets may be employed in fittings for low pressure systems and for systems where purity of the fluid being transported is not adversely affected. However, nonmetallic fittings may not provide adequate strength for high pressure fluid applications and may permit diffusion of atmospheric moisture, oxygen and other contaminants into the high pressure gas stream. Additionally, nonmetallic washers or gaskets may wear quickly and generate wear debris which may be transported into the high purity fluid stream. Furthermore, nonmetallic fittings, especially those with a low melting point, are almost certain to fail in a fire, and hence could yield a catastrophic release of high pressure gas. Thus, fittings for high pressure, high purity fluid systems typically will avoid or minimize the use of nonmetallic materials.
Metallic fittings generally can provide a virtually leak-free connection that prevents escape of high pressure gas from the fitting and that similarly prevents diffusion of atmospheric contaminants into the gas stream, or leakage into vacuum lines. However, the forces applied between the mating surfaces of the two opposed metallic fitting components must be high and must be within certain specified range. Mating forces in a high pressure gas fitting that are too low may permit escape of the high pressure gas and/or may permit diffusion of atmospheric contaminants into the high pressure stream. Conversely, fittings that are connected with a mating force that is too high are likely to accelerate wear due to friction between the assembled components of the fitting, and may cause sealing surfaces to distort or break down after several cycles due to excessive contact pressure.
Mating pressures within a specified range typically are achieved by tightening a fitting with a torque wrench or by tightening a fitting to a specified range of motion. The torque required for proper sealing depends, in part, upon the coefficient of friction between the surfaces in the fitting that slide relative to one another. The torque required to achieve an effective seal in a high pressure, high purity fluid system can be achieved relatively easily in fittings that do not require repeated connection and disconnection. However, the unavoidable rubbing between opposed surfaces can be a significant problem in high sealing integrity systems subjected to repeated connection and disconnection. More particularly, the rubbing of unlubricated metallic surfaces over one another in high pressure fluid systems subjected to repeated connection and disconnection typically will produce wear or galling on one or both of the sliding unlubricated surfaces. This wear often causes the surfaces to become rougher, and thereby leading to an increase in the coefficient of friction. Additionally, wear debris may be generated and may ultimately enter into the fluid stream. Still further, the frictional erosion or galling of interconnected components may eventually lead to gaps between interconnected components with a corresponding leakage of the fluid. Thus, the torque required to achieve an effective seal may increase over the life of the fitting, resulting in further acceleration of wear and related problems as described above. In extreme conditions the rough tightly interconnected components of a fitting may effectively weld together in response to the heat and pressure generated during tightening.
The materials from which the prior art fitting is manufactured must be selected in accordance with their fluid capability. For example, strength in the presence of high pressure, corrosion resistance, chemical inertness, machinability and cost all are important factors. Force bearing characteristics and frictional resistance generally are of secondary consideration. Exotic alloys that meet all requirements might conceivably be available. However, these alloys are very expensive and impose a substantial cost penalty on the manufacturer.
The prior art has attempted to deal with galling by positioning ball bearings between interfaces that are likely to be subject to frictional wear. A joint with such ball bearings is shown, for example, in U.S. Pat. No. 5,118,141 which issued to Miyashita on Jun. 2, 1992. The ball bearings and accompanying races add significantly to the cost and size of the fitting and can complicate manufacturing and assembly processes. The limited load carrying capability of a ball bearing may also prevent conventional metallic seal geometry, with an adverse effect on cost and reliability.
The prior art has also attempted to use dry film lubricants, such as molybdenum disulfide, to reduce wear and control friction. However, these dry film lubricants will wear away in response to high pressure and repeated connection and disconnection. Additionally, the tiny dust-like particles which comprise the dry film lubricants create the potential for contaminating the fluid stream.
In view of the above, it is an object of the subject invention to provide a high pressure fitting with increased wear resistance and a longer life.
It is another object of the subject invention to provide a cost effective fitting for repeated connections and disconnections in a high pressure fluid system.
A further object of the subject invention is to provide a long-lasting fitting without significantly increasing the dimensions required for the fitting.
Still another object of the subject invention is to provide a high seal integrity fitting for repeated connection and disconnection without complicating the connection and disconnection procedure.
Yet another object of the subject invention is to minimize torque that could produce rotary sliding on the sealing surfaces.